Frequency discriminator circuits



Dec. 30, 1947. J. s. LE GRAND ETAL FREQUENCY DISCRIMINATOR CIRCUITS Filed D eC. 20, 1943 IN VEN TORS A TTRNE Y Patented Dec. 30, 1947 UNITED STATES; PATENT GFFI'CE FREQUENCY'DISCRIMINATOR CIRCUITS` Jesse-S. Le Grand and Edwardlia Bussire, New

York, N.. Y., assignors to Federal Telephone and Radio Corporation-,New York, N. Y., a corporation of Delaware ApplcationDecember 20, 1943, Serial No. 5153016 claims. lol. 25o-27) ThisY inventionV relates toimprovementsv in. fre-- quency discriminator circuits, and more: particu.- larly to such circuits as are adaptedto be used` with frequency modulation receivers.

An object of this invention is to provide a frequency discriminator circuit which is satisfactory atvery highv frequencies as well as at the morefusual, lower frequencies.

Morespeciflcally, the invention provides a fre.- quency discriminator circuit ink which the diode' load resistors are not shunted byy any discriminator. or inputv circuitcondensers, and which will remain completelybalanced..

These and other features of the invention will behest understoodand appreciated from the. following description of a preferred embodiment and a` modification thereof, described for pur:-

posesof illustration and shown in the accom..- panying drawings, in'which:

Fig. l isa circuit diagram of a preferred fornil offrequency discriminator in accordance with the pres ent invention;

` Fig. 2 is a circuit diagram of a modified. form offrequency discriminator.

While the use of frequency discriminator circuits embodying apair of diodes ora doubler di-A ode is common practice inA frequency modulation receivers, such circuits as heretofore proposed are unsuitable for the recoveryof high frequency modulation frequencies from frequency' modu lated waves. Allfrequency discriminatorcircuits asA heretofore proposed, embody in the frequency discriminator input coupling circuit, a condenserv which will be in shunt with at least one' of the diodey resistors through the common connection between the midpoint of. the secondary Winding ofthe. input coupling and.` the common ends of the diode resistors. If. this condenser is made of low capacity, loss of efficiency occurs dueto the reactance of the condenser becoming large at the carrier frequency. A radio frequency choke serially connected With this condenser is unsuitable at higher frequencies', while if a large resistor is used instead of this choke, loss of ef.- ciency occurs due to the fact that thevalue of this resistor becomes comparable in value to that of the dioderesistors'.

The frequency discriminator circuit according to the present invention, anda preferred example of which is illustrated. in Eig. 1', overcomes thek aforementioned diflculties. As there illustrated, the frequency discriminator input coupling or transformer I0 has its primary and secondary windings coupled and one end of its secondary Winding connected to the anode I2 of one diode and4 theV cathode I4" of. a second diode, both di odes preferably being. embodied in a double diodev tube |16 of any suitable well-known type. The

cathode I4 of the one diode is connected toy the input, as from the prior intermediate fre-- quency or limiter stage, will be. connected to the center tap ofthe secondary Winding II of the coupling Ill as well as tothe tuned primary cir cuit comprising condenser 8 and primary wind-'1 ing 9. In this case, however, this input maybe connected through a blocking condenser 2.1 prior to the interconnection to the coupling It.. By grounding the common center terminal between the resistors I8 and 2D, it. will thus. be seeny that.

neither the condenser 21 nor any other condenser in the input coupling discriminator circuit Will be in shunt with either of these diode resistors. The connection back to thecenter tap of the secondary Winding. II ofV the coupling transformer Il) will takeplace through the common ground connection, as through the primary winding 9 of the coupling. Thus, the only ca pacitance across the diode. resistors will bel that resulting from the inherent capacity of the-tube and possible wiring capacities. All external capacitors have been eliminated. The inverter typeof circuit shown in. Fig. 1 will result in voltagef drops across the resistor I8 and 2E) whose sumy is measured by the balanced output circuit consisting of condensers 22 and 24 and resistors. 46l and 48', this output being applied to the grids; and 42 of the double triode 26 having theirv cathodes 50 and 5'2" connected together and to ground through resistor 64' and condenser B8 andv anod'esl 44 andv 48 suppliedby a source of anode potential, from Whose output this sum is translated into suitable audio frequency signals.

The modified form of invention shown in Fig.. 2 is identical with the principles of the inventionV illustrated in Fig. 1, but in this case the secondary winding II of the input coupling I0 is connected across the anodes I2 and I2 of the double diode I6, While the cathodes I4 and I4 are connected to ground through resistors 28 and 30 respectively. The blocking condenser 21 is still so positioned that it, or any other discriminator circuit condenser, will not be in shunt with the resistors 28 and 30, the lower end of the primary winding 9 of the coupling I0 being also grounded, as heretofore described. In the form of system illustrated in Fig. 2 the diierence between the Voltage drops across resistors 28 and 30 is measured by the balanced output circuit consisting of condensers 32 and 34 and resistors 36 and 38, this output being applied to the grids 40 and 42 of the double triode 26 having anodes 44 and 48 connected to a source of anode potential and, connected for equal amplitude amplications but for 180 phase shift in one section, and 0 phase shift in the other. This arrangement comprises connecting cathodes 50 and 52 serially through resistors 54 and 56, their midpoint being connected to ground, a bypass condenser being connected across resistor 58, and will result in a completely balanced output taken from cathode 50 through condenser 60 and anode 48 through condenser 62, while at the same time the only capacity in shunt with the diode resistors 28 and 30 will be the inter-tube capacities and the external wire capacity, if any.

While we have described above the principles of our invention in connection with a specic circuit and a modication thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of our invention as set forth in the objects and the accompanying claims.

We claim:

1. Frequency discriminator circuit including, in combination, input coupling means comprising an input transformer having primary and secondary windings, means galvanically connecting one end of said primary winding to the substantial midpoint of said secondary winding, a condenser in shunt with each winding and forming with each a tuned circuit, a first resistor, iirst rectier means serially connecting one end of the secondary winding t one end of said resistor, a second resistor, second rectier means serially connecting the other end of said secondary winding to one end of said second resistor, means having negligibly low loss interconnecting the other end of the primary winding with the other end of the two resistors, and means deriving and comparing the potentials across said two resistors.

2. Frequency discriminator circuit including, in combination, input coupling means comprising an input transformer having primary and secondary windings, means galvanically connecting one end of said primary winding to the substantial midpoint of said secondary winding, a condenser in shunt with each winding and forming with each a tuned circuit, a rst resistor, rst rectifier means serially connecting one end of the secondary winding to one end of said resistor, a second resistor, second rectier means serially connecting the other end of said secondary Winding to one end of said second resistor, means galvanically comparing the potentials tron discharge devices each having an anode, a

cathode and a grid with their grids coupled respectively to one end of each of said resistors and their anodes directly connected with each other and a, combining output circuit.

3. The combination according to claim 2, in which the means interconnecting the other end of the primary winding with the other ends of the two resistors comprises a common ground.

4. Frequency discriminator circuit including, in combination, input coupling means comprising an input transformer having primary and secondary windings, means galvanically connecting one end of said primary winding to the substantial midpoint of said secondary winding, a condenser in shunt with each winding and forming with each a tuned circuit, a rst resistor, rst rectier means having an anode and a cathode, means connecting said anode to one end of said secondary winding and said cathode to one end of said resistor, a second resistor, second rectier means having an anode and a cathode, means respectively connecting the cathode of the second rectifier means to the other end of said secondary winding and the anode to one end of the second resistor, means grounding the other end of said primary winding and the other ends of said two resistors, and translating means responsive to the sum of the voltages across the two resistors including a pair of electron discharge devices each having an anode, a cathode and a grid with their grids coupled respectively to the one end of each of said resistors, their cathodes directly connected with each other and their anodes directly connected with each other and to the output.

5. Frequency discriminator circuit including, in combination, input coupling means comprising an input transformer having primary and secondary windings, means galvanically connecting one end of said primary winding to the substantial midpoint of said secondary winding, and a condenser in shunt with each winding and forming with each a tuned circuit, a first resistor, rst and second rectifier means each having an anode and a cathode, a pair of resistors, means respectively connecting the anodes of said two rectifier means to opposite ends of said secondary winding, means respectively connecting the cathodes of said two rectifier means to one end of said resistors, means grounding the other end of said primary winding and' the other ends of said resistors, and translating means responsive to the diierence between the voltage across the resistors including a pair of electron discharge devices each having an anode, a cathode and a grid with their grids coupled respectively to the one end of each of said resistors, a third and fourth resistor having one end of each connected respectively to the cathodes of said devices and the other end of each connected to each other and to ground, and means coupling the anode of one of said devices to the cathode of the other of said devices and to the output.

JESSE S. LE GRAND. EDWARD LA BOSSIERE.

REFERENCES CITED The following references are le of this patent:

UNITED STATES PATENTS of record in the l 

